GENEVA, May 19 (Reuters) - Cholera has broken out in the capital of South Sudan where five months of civil war has left thousands homeless and disrupted food supplies and health services, according to the World Health Organization.

WHO spokesman Tarik Jasarevic said on Monday nine people were believed to have died of cholera, which can kill in days if not treated. An estimated 138 had been registered so far in and around Juba.

A WHO spokeswoman in Juba told Reuters by phone:

"The outbreak has just started and it's spreading...Although they are not confirmed, we've got reports that other people in other areas are equally affected, so it is certainly spreading."

UNICEF has supplied tents, hygiene equipment, clean water and oral rehydration solutions for a treatment centre. Hundreds of people have been trained over the past 24 hours to inform and mobilise communities, the agency said in a statement.

The outbreak began days after South Sudan's army and rebel forces signed a ceasefire to try to stop five months of conflict.

U.N. Secretary General Ban Ki-moon said earlier this month that if fighting continued, half the population of 12 million will be fleeing, starving or dead by the end of the year.

But aid agencies say even a ceasefire that holds will not immediately resolve dire humanitarian situation in the country.

Three of the deaths were at a health facility and six were in the community, she said.

WHO health security chief Keiji Fukuda, speaking on the sidelines of the World Health Assembly in Geneva, said he did not have details of the latest situation, but added:

"Like any sort of outbreak, particularly where you have one in a difficult area, (you have) to figure out what's going on quickly but also get the right kind of help and support which is needed there."

Cholera is an extremely virulent disease of the small intestine, often accompanied by severe nausea and diarrhoea. It can kill within hours if left untreated, but up to 80 percent of cases can be successfully treated with oral rehydration salts, the WHO says.

The U.N. children's agency UNICEF said 80,000 people in South Sudan had been fully vaccinated against cholera, but still the caseload was doubling every day. (Reporting by Tom Miles, Editing by Ralph Boulton)